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Old 04-01-2006, 09:13 AM
rondahart rondahart is offline
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gas leak at choke lever

While attempting to start my Atomic four, the engine turns over, but does not fire. I have a gas leak at the choke lever. Aside from the obvious leak or seal issue, does this indicate that I have a gas blockage somewhere? What would you recommend?
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Old 04-02-2006, 12:52 PM
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jhwelch jhwelch is offline
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It sounds like your fuel float shutoff in the carb. may be sticking
in the open position and letting gas flood the air intake area.
If you take off the air filter do you see a puddle of gas?

If the needle valve is the culprit take the carb. off the motor
and apart. All it takes is a very tiny piece of grit to cause
this to happen (speaking from too much experience).

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Old 04-02-2006, 01:38 PM
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Don Moyer Don Moyer is offline
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Fuel leaking from the carburetor is usually indicative of a defective or dirty float valve; however, this condition would not normally prevent starting. Here is a checklist that should be helpful in getting the engine started, followed by a rather lengthy tech note on leaky float valves:

The needle of the float valve is actually always being pressed up against the seat by the buoyancy of the floats. As the fuel level lowers, the pressure against the seat is somewhat less, and the pump is able to force some more fuel into the float chamber. As the fuel level rises, the buoyancy once again is sufficient to close off the fuel flow, in opposition to the pressure from the pump.

The fact that the needle is always being pressed against the seat explains why a piece of dirt will remain held between the needle and the seat once it works its way in.

The best safeguard against a leaky float valve is to install a good primary fuel filter with a water separator and then to service it regularly. We also recommend a small inline filter between the fuel pump and the carburetor. Primary fuel filters usually have a 10 micron mesh filter element, while the elements in small inline filters are usually 1 or 2 micron. These small inline filters serve a secondary or "polishing" function.

The dirt can of course be cleaned out by removing the carburetor and disassembling it. While the carburetor is disassembled, the seat of the float valve can be polished by taking a short length of 3/16" wooden dowel rod (bluntly pointed on the end) and pressing it against the seat while rotating it back and forth. This dressing action sometimes assures a better seating of the needle.

Depending on available access, it's sometimes possible to get rid of a piece of trapped dirt in the float valve by draining all the fuel from the carburetor as follows, without actually removing the carburetor from the engine:

1) Remove the ˝" hex-headed main passage plug and drain all of the fuel in the carburetor into a clean glass jar to check for any sign of turbidity in the fuel. The main passage plug lies horizontally in the bottom of the carburetor with the hex-head pointing directly away from the flywheel end of the engine. The main passageway is the lowest part of the carburetor so any crud that is trying to pass through the carburetor will wash out into your clean glass jar. CAUTION: The main passage plug has fine machine threads and seals with a hard black washer under the hex-head. Be careful not to drop the washer when removing the plug, and do not over-tighten the plug when reinstalling it or the washer will extrude out from under the hex-head.

2) While the main passage plug is still removed, it's a good idea to operate the fuel pump to move a couple pints of fuel through the carburetor and into your clean jar (or until the fuel is perfectly clear - whichever comes first). Mechanical pumps have a priming lever which makes this part of the job easy, but electric pumps will usually have an oil safety switch in their circuit making it necessary to connect a temporary jumper wire run between the positive terminal of the coil and the pump so that the pump will operate anytime that the ignition switch is turned on.

NOTE: A convenient way to run an electric fuel pump during troubleshooting or maintenance operations is to procure a hand held push-button starter switch at your local auto parts store and connect it between the big battery cable on the starter solenoid and the terminal on the oil safety switch to which the fuel pump connects. You can then run the pump by simply pushing the handheld starter button

3) Reinstall the main passage plug and work the fuel pump again until the carburetor is charged with fuel.

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Old 04-02-2006, 02:06 PM
rondahart rondahart is offline
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Thank you Don, will give this a shot.

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Old 04-02-2006, 02:33 PM
rondahart rondahart is offline
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Thank you Jonathan, I will consider this during troubleshooting.

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Old 09-07-2021, 11:43 AM
rlv rlv is offline
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About the new late-model carb leaking fuel

I recently installed one of your "new" carbs on my Atomic 4 engine.
This carb started up and ran much better than the previous carb!

However, there was a slow fuel leak (gasoline) coming from the o-ring connection between the air intake flame arrester and the main body.
After some study, I see that the surface treatment of the main body has changed, and is a rough kind of burnished metal that the gasket has trouble sealing against. I've circled the area in pink, the left unit is the new one. On the right side of image you can see the surface is smoother. This would seal better and give the system some more leeway for scavenging.

See image:

One possible solution is to use some sandpaper (600?) and smooth off that area. Also, as shown in your A4 video, the root cause of carb fuel buildup is the float valve sticking or clogging.

But will these activities void my warranty on the new carb?
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Old 09-07-2021, 12:56 PM
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Call Ken at MMI and ask him these questions.
Fuel should not be pooling in the carburetor throat.
Keep the valve(s) between the tank and the engine turned off except when the engine is being used.


EDIT: A likely scenario is the carburetor was correct as shipped from MMI but a bit of crud in the fuel got into the metering needle area and is holding the needle open a bit.

Last edited by JOHN COOKSON; 09-07-2021 at 01:22 PM.
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Old 09-07-2021, 11:15 PM
rlv rlv is offline
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more leeway for carb overload

Yes, I agree, its best to address the root of the problem.
However my identification of the poor seal, where it should
give the system some more leeway when the overload of fuel presents itself is valuable.
I believe that was the goal in implementing the scavenger pipe.

Thanks for helping me though this challenging adjustment.
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Old 09-10-2021, 09:35 PM
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Al Schober Al Schober is offline
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As simple a task as replacing the old carb with the new can by itself cause a problem. Picture this: Remove fuel hose from old carb, barb fitting tears a hunk loose from the old hose, install new carb, attach fuel hose, loose hunk goes into the float valve. New carb floods and leaks gas.
Personally, I recommend hose fitting without the barbs.
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